The present invention relates to a method for substantially nickel-free phosphating of a metallic surface, to a corresponding phosphating composition, and also to a correspondingly phosphate-coated metallic surface.
Phosphate coatings on metallic surfaces are known from the prior art. Such coatings serve to prevent corrosion of the metallic surfaces and also, furthermore, as adhesion promoters for subsequent coating films.
Such phosphate coatings are employed in particular in the areas of the automobile industry and also of general industry.
The subsequent coating films, as well as powder coatings and wet paints, are, in particular, cathodically deposited electrocoat materials (CEC). Since the deposition of CEC requires a flow of current between metallic surface and treatment bath, it is important to set a defined electrical conductivity in the phosphate coating in order to ensure efficient and uniform deposition.
Phosphate coatings, therefore, are customarily applied using a nickel-containing phosphating solution. The nickel deposited in this process, elementally or as an alloy constituent, e.g., Zn/Ni, provides appropriate conductivity of the coating in the course of the subsequent electrocoating.
On account of their high toxicity and environmental harmfulness, however, nickel ions are no longer a desirable constituent of treatment solutions, and ought therefore as far as possible be avoided or at least reduced in terms of their amount.
The use of nickel-free or low-nickel phosphating solutions is indeed known in principle. It is limited, however, to particular substrates such as steel.
The stated nickel-free or low-nickel systems, moreover, may result in poor corrosion values and coating adhesion values under prevailing CEC deposition conditions, owing to a nonideal substrate surface.